1. Field of the Invention
This invention relates to an elastic surface wave device utilizing a parametric amplification effect, and more particularly to an elastic surface wave device having input and output electrodes juxtaposed on the same side in relation with a pump electrode and having a multi-strip coupler interposed between said input and output electrodes and said pump electrode for deriving, through the output electrode, a backward wave signal emanating from the pump electrode to improve separation of an output signal from an input signal.
2. Description of the Prior Art
The applicant has already proposed and disclosed an elastic surface wave device having various advantages which allow an integrated formation as illustrated in FIG. 1, continuous wave operation, improvement in low frequency characteristics, etc. in Japanese patent application No. 107271/77.
In FIG. 1, numeral 1 designates a body which is formed of a laminate of a semiconductor layer 1' and a piezoelectric layer 1". Numerals 2 and 3 designate an input electrode and an output electrode, respectively, and a pump electrode 4 is provided therebetween. Numeral 5 is an ohmic electrode, 6 a d.c. power source for applying a d.c. bias voltage, and 7 a high-frequency power source for applying a pump power.
As mentioned above, in this proposal, the pump electrode 4 is disposed between the input electrode 2 and the output electrode 3 and a surface signal wave (forward wave) passed through and amplified by the pump electrode 4 is taken out through the output electrode 3. The output waveform of this device is as illustrated in FIG. 2. In the figure, a frequency Fp/2 (Fp: a frequency of the pump power) is a center frequency of an output signal. An input signal component 8 having a frequency deviated from the center frequency Fp/2 is propagated as it is without being subjected to a parametric amplification and outputted from the output electrode 3 together with the parametrically amplified component. Due to this fact, in the previously proposed device, an amplification gain G of the parametrically amplified signal 9 and the spurious response R make little difference in magnitude and an absolute value of the amplification gain G cannot be large enough. By these reasons, the selectivity of the output signal cannot be excellent and therefore an improvement in this respect has been up for solution.